User’s Manual of
Spanning Tree Algorithm and protocol are a complicated and complex subject and must be fully researched and understood. It is possible to cause serious degradation of the performance of the network if the Spanning Tree is incorrectly configured. Please read the following before making any changes from the default values.
The Switch STP performs the following functions:
。Creates a single spanning tree from any combination of switching or bridging elements.
。Creates multiple spanning trees – from any combination of ports contained within a single Switch, in user specified groups.
。Automatically reconfigures the spanning tree to compensate for the failure, addition, or removal of any element in the tree.
。Reconfigures the spanning tree without operator intervention.
Bridge Protocol Data UnitsFor STP to arrive at a stable network topology, the following information is used:
。The unique Switch identifier
。The path cost to the root associated with each switch port
。The por tidentifier
STP communicates between switches on the network using Bridge Protocol Data Units (BPDUs). Each BPDU contains the following information:
。The unique identifier of the Switch that the transmitting Switch currently believes is the root Switch
。The path cost to the root from the transmitting port
。The port identifier of the transmitting port
The Switch sends BPDUs to communicate and construct the
The communication between switches via BPDUs results in the following:
。One Switch is elected as the root Switch
。The shortest distance to the root Switch is calculated for each Switch
。A designated Switch is selected. This is the Switch closest to the root Switch through which packets will be forwarded to the root.
。A port for each Switch is selected. This is the port providing the best path from the Switch to the root Switch.
。Ports included in the STP are selected.
Creating a Stable STP TopologyIt is to make the root port a fastest link. If all switches have STP enabled with default settings, the Switch with the lowest MAC address in the network will become the root Switch. By increasing the priority (lowering the priority number) of the best switch, STP can be forced to select the best switch as the root Switch.
When STP is enabled using the default parameters, the path between source and destination stations in a switched network might not be ideal. For instance, connecting